FIELD OF THE INVENTION
The present invention relates to optical telecommunication systems and more particularly to passive optical telecommunication systems which provide integrated services digital networks that comprise a star structure of light waveguides in the level of the subscriber lines. Such integrated services digital networks equipped with light waveguides are basically suitable for narrow band and broadband services, as disclosed in the German patent 24 21 002, corresponding to U.S. Pat. No. 3,980,831, fully incorporated herein by this reference.
Insofar as electrical front-end equipment are not provided, a subscriber-associated light waveguide subscriber line is provided between a subscriber location and a switching center in a pure star network of light waveguide subscriber lines. The expense involved with subscriber-associated light waveguide subscriber lines which extend between subscriber locations and a switching center can be reduced by light waveguide bus networks (potentially topologically appearing as a star network), whereby a respective plurality of decentralized equipment, for example, subscriber-associated network terminations or network terminations respectively shared by a plurality of subscribers that are located directly at the subscriber or at a distance therefrom in the front end of the traffic, is respectively connected via its own light waveguide central office line to an optical brancher that is connected via a light waveguide bus to a common light waveguide terminal of the switching center, either directly or by way of at least one further optical branch. In this connection, one is referred to, for example, the publication ISSLS'88, Conference Papers 9.4.1-9.4.5; Br. Telecom Technol. J. Vol. 7, No. 2, 1989, pp. 100-113, FIG. 5; WO-A1- 88/05233; and WO-81-89/09518.
In such a passive optical telecommunication system in which a respective plurality of decentralized equipment (distant units) is respectively connected via its own light waveguide central office line to an optical brancher that is, in turn, connected either directly or via at least one further optical brancher to a common light waveguide terminal of the central switching system via a light waveguide bus, the signal transmission from the switching center to the decentralized equipment can proceed continuously in a time-division multiplex (TDM) burst stream from which each decentralized equipment accepts only those bursts that are transmitted in time channels allocated accurately to these decentralized equipment, whereas the signal transmission from the decentralized equipment to the switching center proceeds in accordance with a time-division multiple access (TDMA)method in which a decentralized equipment is capable of transmitting its bursts in a time channel reserved for the decentralized equipment in a synchronized fashion with the assistance of the delay device that is set from the switching center. In this connection, one may refer to the publication ISSLS'88, op. cit, p. 9.4.2 and WO 88/09093. To that end, it is known that, based on the prescription of the loop running time between the decentralized equipment and the switching center, the switching center periodically sets a programmable, digital delay device in the decentralized equipment such that the loop running time has a prescribed value with which the bursts transmitted from the decentralized equipment lie exactly in the time channel reserved for this decentralized equipment (WO-A1-88/05233 and WO-A1-89/09518), whereby the setting can be undertaken in two stages in which, first of all, a rough delay having an accuracy of +1 bit (50 ns.) is set, this being followed by a fine delay having an accuracy of +1/10 bits (see Br. Telecom Technolog. J., op. cit, pp. 100 and 108).
A fine delay implemented in the decentralized equipment in addition to the rough delay, for example with the assistance of a variable, analog delay line, involves a corresponding circuit-oriented expense and the present invention discloses a manner of avoiding this expense.